Environmental and temporal patterns in bioturbation in the Cambrian-Ordovician of Western Newfoundland.

The early Paleozoic emergence of bioturbating (sediment-dwelling and -mixing) animals has long been assumed to have led to substantial changes in marine biogeochemistry, seafloor ecology, and the preservation potential of both sedimentary and fossil archives. However, the timing of the rise of bioturbation and environmental patterns in its expansion have long been subjects of debate-resolution of which has been hampered, in part, by a paucity of high-resolution bioturbation data or of systematic investigations of facies trends in lower Paleozoic bioturbation. To address these issues, we conducted an integrated sedimentological and ichnological characterization of the Cambrian-Ordovician Port au Port succession and Cow Head Group of western Newfoundland, encompassing over 350 meters of stratigraphy logged at the centimeter to decimeter scale. We find that, across a wide range of marine facies, bioturbation does not on average exceed moderate intensities-corroborating observations from other lower Paleozoic successions indicating that the early Paleozoic development of bioturbation was a protracted process. Moreover, bioturbation intensities in the Port au Port succession and Cow Head Group are commonly characterized by considerable variability at even fine scales of stratigraphic resolution and changes in bioturbation intensity correlate strongly with variability in sedimentary facies. We observe that facies recording nearshore depositional environments and carbonate-rich lithologies are each characterized by the highest intensities of both burrowing and sediment mixing. These data highlight the need for a high-resolution and facies-specific approach to reconstructing the evolutionary history of bioturbation and suggest that average levels of bioturbation, although relatively low throughout this interval, increased notably earlier in nearshore marine settings.

Revisiting the sedimentary record of the rise of diatoms.

Diatoms are a major primary producer in the modern oceans and play a critical role in the marine silica cycle. Their rise to dominance is recognized as one of the largest shifts in Cenozoic marine ecosystems, but the timing of this transition is debated. Here, we use a diagenetic model to examine the effect of sedimentation rate and temperature on the burial efficiency of biogenic silica over the past 66 million years (i.e., the Cenozoic). We find that the changing preservation potential of siliceous microfossils during that time would have overprinted the primary signal of diatom and radiolarian abundance. We generate a taphonomic null hypothesis of the diatom fossil record by assuming a constant flux of diatoms to the sea floor and having diagenetic conditions driven by observed shifts in temperature and sedimentation rate. This null hypothesis produces a late Cenozoic (∼5 Ma to 20 Ma) increase in the relative abundance of fossilized diatoms that is comparable to current empirical records. This suggests that the observed increase in diatom abundance in the sedimentary record may be driven by changing preservation potential. A late Cenozoic rise in diatoms has been causally tied to the rise of grasslands and baleen whales and to declining atmospheric CO2 levels. Here we suggest that the similarity among these records primarily arises from a common driver-the cooling climate system-that drove enhanced diatom preservation as well as the rise of grasslands and whales, rather than a causal link among them.